[GHC] #12680: Permit type equality instances in signatures

[GHC]

#12680: Permit type equality instances in signatures
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 There isn't a specific spot. An equality between skolems would end up in
 `TcCanonical.canEqTyVarTyVar` which just builds the equality. Because
 tyvar equalities are never absolutely insoluble, the skolem case isn't
 separated out.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12680#comment:18>
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